This invention relates to a process for the preparation of bright, color-pure iron oxide red pigments by the precipitation of iron(II) salts with alkali in aqueous solution, oxidation at temperatures of from 20.degree. C. to 100.degree. C., followed by annealing of the resulting precipitation products in air to iron oxide red pigments, and the use of these iron oxide red pigments.
There are basically four known processes for the preparation of synthetic iron oxide red. One is the oxidation of iron oxide black Fe.sub.3 O.sub.4 (T. C. Patton, Pigment Handbook, Volume 1, John Wiley and Sons, 1973).
Iron oxide red pigments are used in building materials and lacquers. Bright, color-pure iron oxide red pigments are desirable in particular for use in lacquer systems. The color shade of the pigment in the lacquer is determined according to DIN 6174 (equivalent ISO/DIN 7724, 1-3 drafts). The red content, (a*)AS determined in the lacquer is taken as a measure of the color purity of the iron oxide red pigment.
The color shade of the iron oxide red pigment obtained after annealing is influenced by the choice of starting material, namely needle-shaped goethite or isometric magnetite. The use of needle-shaped goethite results in red pigments which have an undesirable yellow tinge. Finely divided isometric magnetites give rise to an iron oxide red with a purer color on annealing. Finely divided magnetites, however, can only be obtained by the aniline process, using aromatic nitro compounds as oxidizing agents.
Commercial iron(II) salts of the kind obtained as waste liquors in steel pickling works and titanium dioxide factories are normally used as the raw material for obtaining precipitated magnetite as the starting material for iron oxide red pigments. These waste liquors normally contain a small, variable proportion of iron(III). Precipitation is carried out, for example, with NaOH, Na.sub.2 CO.sub.3, MgO, MgCO.sub.3 or NH.sub.3. The oxidizing agent used for precipitating the magnetite may be oxygen, air, chlorates, nitrates, peroxides, etc.
Three processes are known for the precipitation of isometric magnetites, the one-stage process, the two-stage process and the iron(III) chloride process.
In the one-stage process (Winnacker-Kuchler, Volume 2, page 171, Munich 1970), iron oxide black is prepared in one step from iron(II) salts and alkalies with gassing at temperatures above 50.degree. C. and pH values above 4.5. The iron(II) salt used in the process should be as far as possible free from iron(III) constituents (DE-PS No. 2,919,854, page 8, lines 19 et seq.).
In the first step of the two-stage process (Winnacker-Kuchler, Volume 2, page 171, Munich 1970), a yellow nucleus is precipitated at an acid (pH ph &lt;5) or an alkaline pH above 12 with alkalies at temperatures from room temperature to 100.degree. C. under oxidizing conditions. This time-consuming step is followed by the second stage of the process, consisting of reacting the yellow nucleus with a further quantity of iron(II) salt and alkalies to form iron oxide black.
The iron(III) chloride process for the precipitation of iron oxide black is technically particularly complicated. It starts with mixtures of iron(III) salt and iron(II) salt used in approximately stoichiometric proportions corresponding to that of the magnetite, i.e. about 1/2 to 2/3 of the mixture is of the expensive iron(III) salt. The iron(III) used in this process is preferably iron(III) chloride (JP No. 56-60 789), which is highly corrosive. This salt is precipitated with alkalies to form brownish-black iron oxides.
The precipitation processes mentioned above yield magnetites which, on annealing, give rise to iron oxide red in the form of medium-fine to coarse iron oxide red pigments. These processes have the disadvantage that they do not yield the coloristically valuable iron oxide red pigments in the bright, color-pure range.
In order to obtain bright, color-pure iron oxide red pigments by the annealing of precipitated magnetite, it is necessary to use a precipitating magnetite which has a crystallite size of about 30 to 50 nm determined by X-ray and a specific surface area S.sub.BET of from 25 to 60 m.sup.2 /g. Magnetites having a particle size or crystallite size in this range cannot be obtained by the three precipitation processes mentioned above.
Various procedures are known for enabling products in the required range of crystallite sizes and BET surface areas to be obtained from the precipitation of iron oxide black. These procedures include reduction in the reaction temperature and lowering of the pH during precipitation, but these only give rise to a non-uniform precipitation product of isometric magnetite and needle-shaped components such as .alpha.-FeOOH and .gamma.-FeOOH. When these precipitated iron oxides are annealed, they give rise to .alpha.-Fe.sub.2 O.sub.3 of non-uniform particles and with a disturbing yellow tinge.